The present application is the U.S. national stage application of International Application PCT/FI99/00544, filed Jun. 21, 1999, which international application was published on Dec. 29, 1999 as International Publication WO 99/67625 in the English language. The International Application claims the priority of Finnish Patent Application 981443, filed Jun. 23, 1998.
The present invention relates to a method as defined in the claims and to a device as defined in the claims for determining fiber orientation in a paper sample.
In this context xe2x80x98paper samplexe2x80x99 refers to web-like products, such as paper, cardboard or other paper industry products, manufactured from lignocellulose based materials.
xe2x80x98Fiber orientationxe2x80x99 refers to the orientation of the fibers in a paper sample. The fibers are mostly oriented in the machine direction rather than in the transverse direction. In a paper sample, the degree and angle of fiber orientation vary in relation to the web width, i.e. in the transverse direction of the paper web, and in relation to time, i.e. in the longitudinal direction of the paper web. Fiber orientation is generally different in different paper sorts.
Previously known is a method for determining the mean fiber orientation in a paper sample as a ratio of machine direction tensile strength to transverse direction tensile strength, i.e. as a strength ratio. Further, a method for determining mean orientation by measuring the speed of propagation of ultrasound in a web in different directions is known. In this method, the distance lag or sound modular ratio obtained indicates the mean orientation angle and degree. Devices suited for implementing the method are manufactured e.g. by Lorentzen and Wettre and Nomura.
Previously known is also a method for determining mean orientation by optical means. In the method, a thin round beam of light is directed at the paper surface and the shape of the beam is measured from the other side of the paper. The ellipticity of the shape of the beam increases in proportion to the degree of orientation in the sample. Instruments for this purpose are manufactured e.g. by Honeywell.
A problem with the prior-art methods is that only the mean fiber orientation in a certain area in the paper sample can be determined. Prior-art methods for determining fiber orientation are difficult to use in practice; for instance, the tensile strength ratio is not easy to determine. Tensile strength ratio is not a good measure of fiber orientation in other respect, either, because, in addition to fiber orientation, it depends on the drying history of the paper, such as the degree of shrinkage occurring in the web as it is drying.
A further problem is that prior-art methods are not applicable for the determination of surface orientation (surface orientation refers to fiber orientation at the paper surface) and surface orientation difference between surfaces.
However, determination of surface orientation is an important function because, depending on the former type used in the paper machine and on the grammage level of the paper web, the web may have a significant orientation difference between its surfaces. This is a problem especially in multi-layer type web formation, in which the layers of the web are produced using different head boxes/formers or a multi-channel head box. Moreover, a surface orientation difference can be clearly observed in paper produced using conventional head boxxe2x80x94former combinations or dilution head boxes. For example, in fine grade paper and sheeted printing paper, a surface orientation difference and variations of orientation in different areas of the paper surface may cause severe curling of the sheet, which again may result in toppling of paper piles, among other things.
The object of the invention is to eliminate the problems mentioned above and to disclose a new workable method and device for determining fiber orientation that will be easier to implement in industrial applications. A further object of the invention is to disclose a method and device for determining, besides mean fiber orientation, fiber orientation separately for the upper and/or lower surfaces of a paper sample.
The method and device of the invention are characterized by what is presented in the claims.
The invention is based on determining fiber orientation in a paper sample.
According to the invention, the surface of the sample is illuminated from an oblique angle using more than one light source. The illumination is rotated on the surface of the sample by switching the light sources electronically on and off in turn by means of a switch. The intensity of the light reflected from the sample surface is measured by means of at least one sensor and, based on the measured light intensity and momentary direction of illumination, fiber orientation in the sample is determined using a computing device.
Based on the measured light intensity and the momentary direction of illumination, it is possible to determine the directional angle and/or degree of fiber orientation in the sample. The surface of the sample can be illuminated and/or the intensity of the light reflected from the sample surface can be measured continuously.
In an embodiment of the invention, the surface of the sample is illuminated using LED type light sources. If desirable, the sample can be illuminated with polarized or non-polarized light.
In an embodiment of the invention, the intensity of the light reflected from the surface of the sample is measured using a light emitting diode. The light emitting diode is disposed e.g. substantially perpendicularly to the sample surface. In an embodiment, at least two sensors are disposed substantially in the form of a bar, and the bar is placed in a position substantially perpendicular to the direction of motion of the sample.
In an embodiment of the invention, the light sources and sensor are disposed in a substantially unitary assembly, forming a measuring device. In an embodiment, the device comprises a movable controlling element, the measuring device being placed substantially in conjunction with said controlling element. The measuring device as such may be free of movable parts. The controlling element may be designed to be movable by a previously known technique, e.g. using a hydraulic device.
According to an embodiment, the measurement is performed substantially above and below the sample. In an embodiment of the invention, the sample is a moving web.
With the method and/or device of the invention, fiber orientation on different surfaces of the sample can be determined separately so that it will be possible to determine separate surface orientations and a surface orientation difference in addition to the mean fiber orientation. A further advantage of the invention is that it allows on-line type determination of fiber orientation even from a moving sample and without breaking the sample.
A further advantage is an insignificant need for maintenance of the measuring device because the measuring device is completely or partially free of movable parts. A further advantage is a compact size and simplicity of manufacture of the device. The apparatus can be easily added to existing equipment e.g. in paper industry without any substantial changes, thus allowing easy, quick and advantageous implementation of the method.
A further advantage of the invention is reproducibility and stability of the light intensity measured by the sensor.